Oil recovery process



United States Patent 3,251,411 OIL RECOVERY PROCESS Le Roy W. Holm, Crystal Lake, Ill., assignor, by mesne assignments, to Union Oil Company of California, Los Angeles, Calif., a corporation of California No Drawing. Filed July 18, 1962, Ser. No. 210,852 5 Claims. (Cl. 1669) This invention relates to an improved method for recovering oil from underground reservoirs. More particularly, this invention relates to an improved secondary recovery process for displacing substantially all of the petroleum from an underground formation.

The prior art discloses methods for increasing the recovery of oil from petroleum reservoirs by displacing the oil with a solvent which is oil-miscible, and then displacing the solvent with cheap scavenging fluids which may be left in the reservoir. Gas has been employed with considerable success as the scavenging fluid. Liquefied, normally-gaseous hydrocarbons, and especially mixtures of propane and butane, wit-h perhaps minor amounts of ethane and pentane, have been used as oil-miscible solvents with considerable success. US. Patent 2,968,350, to Slobod et 211., discloses a process wherein a slug of natural gas is injected after the injection of liquefied propane, but before the injection of flood water. Outstanding recoveries are said to be achieved by this process when it is carried out at suitable temperatures and pressures such that the methane is miscible with the slug of liquefied normally gaseous hydrocarbon.

It has now been found that equivalent oil recoveries can be obtained in a process which is more economical by reason of the fact that large quantities of methane or natural gas are not required. It is therefore a primary object of this invention to provide an improved secondary recovery process whereby excellent oil recoveries can be obtained without requiring the injection of large amounts of valuable materials. Other objects of this invention will become apparent from the following description.

Briefly, in accordance with this invention, a petroleum reservoir penetrated by an injection well and a producing well is treated by injecting a slug of liquefied, normally gaseous hydrocarbons followed by a slug of nitrogen or hutrqssarssatein n nd driving these ec terials by the injection of flood water towards producing wells. Reservoir fluids are recovered from the producing wells in the conventional manner.

In accordance with this invention, the quantity of liquefied normally-gaseous hydrocarbons first injected will be in the range of 0.02 to 0.20 reservoir pore volume, and preferably about 0.05 reservoir pore volume. The quantity of nitrogen-containing gas will be in the range of 0.05 to 1.0 reservoir pore volume, and preferably about 0.15 reservoir pore volume. The injected gas must contain not less than about 75% by volume nitrogen, the remaining ingredients being materials such as oxygen, carbon dioxide, carbon monoxide, methane, or inert gases. It has been found that air is prefectly suitable for use as the injected gas, and for reasons of economy, air is preferred. Other gases containing at least 75% nitrogen, such as flue gas or engine exhaust, if available at no cost, can be utilized to advantage. Water is injected in the usual manner to drive the first-injected liquefied normallygaseous hydrocarbons and nitrogen-containing gas towards the producing rwell. Additional gas may be injected simultaneously or intermittently with the water after the liquefied hydrocarbons and nitrogen-containing gas, to improve the sweep efliciency of the flood.

The process of this invention has been found capable of producing excellent oil recoveries, but critical to the obtaining of such recoveries is a reservoir temperature within the range of .170 to 250 F. Accordingly, the process of this invention is applicable only to reservoirs which exist at or are brought to a temperature within this range. Also critical to the effectiveness of the method of this invention, is the injection of the liquefied hydrocarbons, nitrogen-containing gas, and flood water at a pressure in the range of 1000 to 1500 p.s.i., and the maintaining of this pressure at the injection well until the solvent bank reaches, or nearly reaches, the producing well. The process is applicable to reservoirs containing oils having a gravity of about 35 AP I or higher.

The excellent oil recoveries obtainable by the method of this invention, as well as the criticality of the conditions of temperature and pressure under which the process is carried out, have been demonstrated by a series of laboratory experiments. Shown in Table I are the oil recoveries obtained by treating a Berea sandstone core having a diameter of 3 /2 inches and a length of 16 feet under conditions set forth in the table. After each experiment, the water and oil phases in the core were reconstituted by lcnown techniques so that at the beginning of each experiment the core existed under the conditions set forth in the table. The oil phase in the core, in each experiment, consisted of 44 API crude oil; while the water phase consisted of a 1.5 wt. percent sodium chloride brine. In each experiment, the core as reconstituted, was first flooded with water to residual oil saturation prior to solvent flooding in accordance with this invention. The purpose of the initial waterflood was to reduce the core to conditions typical of those existing in petroleum reservoirs which have been subjected to a waterflood process which has been carried to the practical economic limit of oil recovery. It is this reservoir condition in which solvent floods usually become economically feasible and in practice are resorted to. However, the process of this invention is not restricted to waterflooded reservoirs, but is applicable also during or after primary production.

The experiments establish the criticality of the conditions specified for the process of this invention. Liquefied propane was used as the liquefied, normally-gaseous hydrocarbon and nitrogen was used as the injected gas, except in experiment four, wherein methane was used. In all experiments, the slugs of solvent and gas were followed by water.

TABLE I Propane- Propane- Propane- Propanenitrogen nitrogen nitrogen methane Experiments at 180 F. at 180 F. at F. at F. and 1,250 and 900 and 1,250 and 1,250 p.s.i.g. p.s.rg. p.s.i.g. p.s.i.g.

Original oil in place, percent PV 59.0 58. 2 59. 0 58.0 Oil recovered by waterflood, percent PV 29. 8 30.1 29. 8 24. 2

Residual oil after waterflood, percent PV 29. 2 28. 1 29. 2 33. 8

Solvent Flood:

Propane injected, percent PV 1 18 18 18 19 Gas injected, percent PV 1 18 18 18 17 Oil recovered at total fluid pore volumes injected, percent 01.13.:

1 Measured at 1,250 p.s.i.g. and 75 F. 2 O.I.P.oi1 in place.

The data shown in column 1 of the above table establish the recoveries which can be achieved utilizing the process of this invention. The data in column 2 establish the criticality of pressure to the process of this invention, and show the great decrease in recovery efiiciency which occurs when the process is carried out at pressures below the range specified. The data in column 3 establish the critica-lity of reservoir temperature on the process of this invention, and show the extent to which the recoveries of the process are diminished when the process is carried out at temperatures below those specified. The data in column 4 are those obtained by carrying out the prior art propane-methane-waterfiood process using substantially the same quantities of materials and at the same temperatures and pressures as those at which the process of this invention is carried out. Thus it is established that the process of this invention, While utilizing cheaper materials, produces recoveries equivalent to or greater than those of the prior art processes.

When the process of this invention is carried out in reservoirs having internal temperatures in excess of about 200 F., it is preferred to maintain the injection pressure not much in excess of 1000 p.s.i. in order to reduce the possibility of spontaneous ignition in the reservoir, at least when air is used as the injected gas. Alternatively, or additional-1y, when treating reservoirs which exist at temperatures much in excess of 200 F., the possibility of spontaneous ignition in the reservoir can be eliminated by reducing the oxygen content of .the injected gases, as by diluting air with the flue gas or exhaust :gas if available, or by reducing the oxygen content of the air to a tolerable level by burning sufficient carbonaceous or hydrocarbon material in the air prior to injection.

'Preferably the oxygen content will be below about 5% where the reservoir temperatures approach 250 F.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The method of recovering petroleum from underground reservoirs existing at a temperature in the range of to 250 F. and penetrated by an input well and a producing well comprising injecting through said input Well and into said formation 0 02 to 0.20 reservoirpore du cingwell.

2. The method in accordance with claim 1 in which said gas is air.

3. The method in accordance with claim 2 in which said hydrocarbon is injected in the amount of about 0.05 reservoir pore volume.

4. The method in accordance with claim 3 in which the amount of said gas injected is about 0.15 reservoir pore volume.

5. The method in accordance with claim 4 in which said hydrocarbon, gas, and floodwater are injected at a pressure of about 1250 p.s.i.

References Cited by the Examiner UNITED STATES PATENTS 2,798,556 7/1957 Binder 1669 2,885 ,002 5/ 1959 Jenks 166-9 2,968,350 l/ 1961 Slobod l6'69 CHARLES E. OC-ONNELL, Primary Examiner. 

1. THE METHOD OF RECOVERING PETROLEUM FROM UNDERGROUND RESERVOIRS EXISTING AT A TEMPERATURE IN THE RANGE OF 170 TO 250*F. AND PENETRATED BY AN INPUT WELL AND A PRODUCING WELL COMPRISING INJECTING THROUGH SAID INPUT WELL AND INTO SAID FORMATION 0.02 TO 0.20 RESERVOIR PORE VOLUME OF LIQUEFIED NORMALLY-GASEOUS HYDROCARBON, THEREAFTER INJECTING 0.05 TO 1.0 RESERVOIR PORE VOLUME OF A GAS CONTAINING NOT LESS THAN ABOUT 75% NITROGEN BY VOLUME, AND DRIVING SAID HYDROCARBONS AND GAS TOWARD SAID PRODUCING WELL BY THE INJECTION OF FLOODWATER, SAID HYDROCARBONS, GAS, AND WATER BEING INJECTED WHILE MAINTAINING THE RESERVOIR AT A PRESSURE IN THE RANGE OF ABOUT 1000 TO 1500 P.S.I., SO AS TO MAINTAIN SAID LIQUEFIED NORMALLY. GASEOUS HYDROCARBON IN THE LIQUID STATE THROUGH A MAJOR PORTION OF THE DISTANCE BETWEEN SAID INPUT WELL AND PRODUCING WELL AND RECOVERING RESERVOIR FLUIDS FROM SAID PRODUCING WELL. 